<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let scoer = Number(prompt('请输入你的成绩'))
      if (scoer >= 90) {
        alert('A')
      } else if (scoer >= 80 && scoer < 90) {
        alert('B')
      } else if (scoer >= 70 && scoer < 80) {
        alert('C')
      } else if (scoer >= 60 && scoer < 70) {
        alert('D')
      } else {
        alert('E')
      }
  </script>
</body>
</html>